1. Field of the Invention
The present invention relates to a battery-powered mobile communication terminal device.
2. Description of the Related Art
In recent mobile communication terminal devices such as cellular phones, battery voltages employed have been lowered as the operational voltages of the internal circuits have been decreased. On the other hand, lithium ion batteries have become widely used because they have an advantage in battery capacity and weight. FIG. 16 is a block diagram showing the configuration of a transmitting section of a conventional mobile communication terminal device; FIG. 17 is an explanatory diagram illustrating a relationship between an input signal and its modulation frequencies; FIG. 18 is an explanatory diagram illustrating the output phase of a baseband signal output from a baseband modulator; FIG. 19 is an explanatory diagram illustrating phase errors included in a carrier wave; FIG. 20 is an explanatory diagram illustrating the output phase of a transmission signal output from a quadrature modulator; and FIG. 21 is a diagram showing a relationship between transmission power control levels and their transmission outputs in a digital portable phone using the GSM system.
Referring to FIG. 16, reference numeral 8 denotes a microphone; 9 denotes a control section; 10 denotes a baseband modulator; 11 denotes a modulation phase generating section; 12 denotes a phase adder; 13 denotes a phase converter; and 14 denotes a D/A converter. The baseband modulator 10 is composed of the modulation phase generating section 11, the phase adder 12, the phase converter 13, and the D/A converter 14. Reference numeral 15 denotes a quadrature modulator for modulating a carrier wave according to a baseband signal and outputting a transmission signal; 16 denotes a transmission local oscillator for receiving a battery voltage supply and generating a carrier wave; 17 denotes a battery for supplying a voltage to each circuit; 18 denotes a voltage regulator for regulating the battery voltage output from the battery 17 to a constant voltage, and outputting it to the transmission local oscillator 16; 19 denotes a transmission power amplifier for amplifying the transmission signal to a predetermined transmission output level; 20 denotes a transmission ON/OFF control section for controlling the ON/OFF states of the transmission power amplifier 19; and 21 denotes an antenna for transmitting the transmission signal amplified by the transmission power amplifier 19.
Two power supply routes are used for driving the mobile communication terminal device shown in FIG. 16. One route directly supplies power from the battery 17 to the transmission power amplifier 19, while the other supplies a regulated, substantially constant voltage to the transmission local oscillator 16 through the voltage regulator 18. To efficiently drive a circuit consuming a particularly large power, such as the transmission power amplifier 19, it is necessary to directly connect the circuit to the battery 17 for power supply. On the other hand, power is supplied through the voltage regulator 18 to circuits such as the transmission local oscillator 16 which do not require large amounts of power but require a stable power supply.
The transmission local oscillator 16 generates a radio frequency wave having a predetermined frequency used as a carrier wave, and the phase and frequency of the carrier wave generated by the transmission local oscillator 16 are modulated by the quadrature modulator 15 according to a baseband signal. Therefore, it is especially important to supply a stable voltage to the transmission local oscillator 16, which generates a carrier wave, in order to ensure communication quality.
Next, description will be made of the operation of the transmission section of the mobile communication terminal device. An analog voice signal entered from the microphone 8 is converted into a digital voice signal (1/0 signal) in the control section 9. Then, the digital voice signal subjected to a voice coding process is arranged in the TDMA (Time Division Multiple Access) transmission format, and output to the modulation phase generating section 11 of the baseband modulator 10. FIG. 17 shows frequencies of the digital voice signal (1/0 signal) after it is passed through a Gaussian filter. Each frequency is equivalent to a variation in phase per unit time, or a microphase, indicated by a hatched portion in FIG. 17.
The modulation phase generating section 11 generates a microphase according to input transmission data, and outputs it to the phase adder 12. Microphases output from the modulation phase generating section 11 are integrated (added by the phase adder 12) into the phase information shown in FIG. 18. The phase converter 13 converts the phase information output from the phase adder 12 into inphase components I and quadrature components Q, namely an I digital modulation signal and a Q digital modulation signal, and outputs them to the D/A converter 14. The D/A converter 14, in turn, converts the I and Q digital modulation signals output from the phase converter 13 into I and Q analog modulation signals, and then outputs them to the quadrature modulator 15 as baseband signals.
The transmission local oscillator 16 receives a battery voltage supply output from the voltage regulator 18 to generate carrier waves, and outputs the generated carrier waves to the quadrature modulator 15. The quadrature modulator 15 modulates the carrier waves according to the I and Q analog modulation signals. Specifically, the two carrier waves which are 90 degrees out of phase with each other (local oscillation signals generated in the transmission local oscillator 16) are each multiplied by one of the I and Q analog modulation signals, and combined to produce and output a transmission signal to the transmission power amplifier 19.
Since a mobile communication terminal device using the TDMA method makes transmission during only assigned time periods (time slots), the transmission power amplifier 19 is controlled so that it is turned ON only during such time-slot transmissions, and turned OFF otherwise. Such ON/OFF control is performed by the transmission ON/OFF control section 20. The transmission signal amplified to a predetermined transmission output level by the transmission power amplifier 19 is output from the antenna 21 as a transmission wave. The transmission output of the transmission power amplifier 19 is controlled according to transmission power control levels, shown in FIG. 21, employed in digital portable phones using the GSM system.
Incidentally, since the battery 17, which supplies power to the mobile communication terminal device, has a limited power supply capacity, the battery voltage supplied to the circuits momentarily (for about a few tens of A.mu. seconds) drops when a large amount of power consumption occurs. Specifically, when the transmission ON/OFF control section 20 turns ON the transmission power amplifier 19 at the time of starting a transmission for a time slot, power consumption occurs to amplify the transmission signal, momentarily lowering the output voltage of the battery 17.
When the battery voltage output from the battery 17 is lowered due to the initiation of operation of the transmission power amplifier 19, the regulator output voltage from the voltage regulator 18, which regulates the battery voltage to a substantially constant voltage, also momentarily drops. Since the transmission local oscillator 16 receives a voltage supply from the voltage regulator 18, a drop in the regulator output voltage disturbs the oscillation frequency of the transmission local oscillator 16, producing a phase error in a generated carrier wave as shown in FIG. 19. When a carrier wave including such a phase error is output to the quadrature modulator 15, and modulated according to a baseband signal, the transmission signal sent from the antenna 21 after it is amplified by the transmission power amplifier 19 also includes a phase error, resulting in deteriorated communication quality, as shown in FIG. 20.
Furthermore, when a lithium ion battery having a large internal impedance is used as the battery 17, or when the battery 17 has been exhausted and as a result, the magnitude of a battery voltage which can be generated itself has been lowered, the magnitude of a voltage drop at the time of starting the operation of the transmission power amplifier 19 increases. Specifically, as the remaining capacity of the battery 17 decreases, the degree of a phase error included in a carrier wave increases, resulting in greater deterioration of modulation accuracy. Accordingly, when the remaining battery capacity has become lower than a predetermined value, it is necessary to replace the battery 17 considering maintenance of communication quality, which is a problem from the viewpoint of efficient use of the battery 17.